1. Field of the Invention
This invention has to do with nickel aluminide intermetallic alloys for metal-forming tooling applications which take advantage of the high temperature strength and wear resistance of these alloys. Specifically, this invention is directed to the elimination or minimization of the nickel zirconium eutectic phase in the cast or wrought tooling through the addition of measurable amounts of molybdenum (Mo) to the nickel aluminide (Ni.sub.3 Al) alloy in order to increase the useful service life of the tooling made from it; thus providing the advantages of increased productivity, enhanced quality and reduced costs in a manufacturing set up.
Also, this invention has to do with the heat treatment or thermal processing of nickel aluminide (Ni.sub.3 Al) intermetallic alloys for use in high temperature applications and tooling for open and closed die forging where high strengths and hardnesses are required but without sacrifice of ductility in order to improve lifetimes of the tooling made from these alloys.
2. Description of the Prior Art
For about ten years, substantial efforts have been devoted to research and development of ordered intermetallics. Ordered intermetallic compounds constitute a unique class of metallic materials that form a long range, ordered crystal structure below a critical temperature, generally referred to as the critical ordering temperature (Tc). These ordered intermetallics usually exist in relatively narrow compositional ranges around simple stoichiometric ratios. Significant progress has been made in understanding their susceptibility to brittle fracture and in improving ductility and toughness of Ni.sub.3 Al at both low and high temperatures. In a number of cases, significant tensile ductility has been achieved at ambient temperatures by controlling ordered crystal structures, increasing deformation modes, enhancing bulk and grain-boundary cohesive strengths, and controlling surface composition and test environments. Success in these areas has inspired parallel efforts aimed at improving mechanical strength properties. The attempts for practical usage of intermetallics were first realized in the field of "functional materials" such as magnetic materials, semiconductor materials and superconducting materials. However, as far as their usage as structural engineering materials is concerned, the intermetallics were completely ignored because of their extreme brittleness and poor ductility. The discovery of the ductilizing effect that boron has on the alloy has led to the expectations for practical usage of the intermetallics as heat resistant materials. The nickel aluminide (Ni.sub.3 Al) of interest in the instant invention has potential as high temperature engineering materials due to its tendency to increase in yield strength and tensile strength with an increase in test temperature.
The alloy design work has been centered primarily on aluminides of nickel, iron and titanium and this work has resulted in substantial improvements in the mechanical and metallurgical properties of these materials at ambient and elevated temperatures. Of particular interest to the instant invention are the aluminides based on nickel which have had to be engineered to overcome ductility problems namely brittle cracking and crazing in order to be ready for structural applications. It has been the perception in the industry that nickel aluminides are so brittle the compounds simply cannot be fabricated into useful structural components. Even when fabricated, these compounds have a low fracture toughness that severely limits their use as engineering materials. The study of the ductility and strength of Ni.sub.3 Al has led to the development of ductile nickel aluminide alloys for structural applications. According to a review article by Oak Ridge National Laboratory scientists, the alloys generally contained hafnium, zirconium, tantalum, and molybdenum at levels up to 8 weight % for improving strength at elevated temperatures. The starting nickel aluminide alloy IC-221M for the instant invention was developed by researchers at Oak Ridge National Laboratories (ORNL) with controlled additions of chromium (Cr), molybdenum (Mo), zirconium (Zr), and boron (B). Both the boron and chromium additions improved the intermediate ductility at room and high temperatures. Molybdenum improved the room and high temperature strength. Zirconium improved high temperature strength, oxide spallation resistance, weldability, and castability. The alloys generally contain zirconium and molybdenum at levels up to 8 weight % for improving strength at elevated temperatures. They contain up to 10 weight % chromium for enhancing ductility at intermediate temperatures of 750.degree. F. to 1650.degree. F. Boron at levels of 0.01 weight % or less is added for strengthening grain boundaries and increasing ductility at ambient temperature.
Several patents related to these structures have been allowed. They are listed below.
Reexamination Certificate issued Jul. 23, 1997 for U.S. Pat. No. 4,612,165, Ductile Aluminide Alloys for High Temperature Applications.
U.S. Pat. No. 4,731,221, Nickel Aluminides and Nickel-Iron Aluminides for Use in Oxidizing Environments.
U.S. Pat. No. 5,006,308, Nickel Aluminide Alloy for High Temperature Structural Use.
U.S. Pat. No. 5,108,700, Castable Nickel Aluminide Alloys for Structural Applications.
U.S. Pat. No. 4,711,761, Ductile Aluminide Alloys for High Temperature Applications.
Researchers at the Institute of Aeronautical Materials in Beijing, China have also developed a castable nickel aluminide (Ni.sub.3 Al) intermetallic alloy. The nominal composition of this alloy is 14 weight % molybdenum and 0.03 to 0.15 weight % boron. Because the alloy was developed for applications in fail-safe environments like gas turbine blades and air transport vanes, this alloy was required to have yield strengths in the vicinity of 120,000 psi, tensile strengths in the vicinity of 183,000 psi and was not allowed to have any measurable amount of zirconium so as to prevent the formation of the nickel-zirconium eutectic phase. Heat checking and cracking occurs in this phase with the resultant failure of the component.
These alloys were limited in their usefulness to the manufacturing and commercial products markets because of a lack of experience and willingness to melt and cast the high aluminum contents found in these nickel aluminides. Using ORNL's "exothermic melt process" and the nickel aluminide alloy IC-221M, successful melt and pour of commercial-sized heats up to 8,000 pounds have been accomplished. However, the forging dies that were cast from these heats were limited in their useful life due to heat checking, thermal fatigue, and cracking of the die material. This heat checking and cracking arose from the nickel zirconium eutectic phase formed between the zirconium, added for improved castability, and the nickel in the nickel aluminide alloy IC-221M. The surface cracks propagated from the surface of the die into the bulk of the die material, negatively impacting the useful life of the die material and causing the work piece to stick to the die surface and in the die cavity. This slows production and leads to the scrapping of the workpiece because of surface indications. As the extent of the heat checking and cracking increased, more time in the die repair shop had to be spent polishing and grinding dies. Upon resink of the die, substantially greater amounts of the die material must be removed before the die can be returned to service. If the heat checking and cracking are severe enough, severe mechanical fatigue and die breakage will occur. The quantity of pieces that could be realized on the die is shorted, as well as, the uptime on the press itself. The higher costs associated with the heat checking and cracking problem on these dies come from higher maintenance requirements of polishing and grinding dies in the press, die breakage, lower production, higher scrap, die material loss, elevated sinking times, and less pieces per die. Consequently, increased productivity, enhanced quality and cost savings in a manufacturing setting were the drivers for the instant invention.
Researchers at Special Metals Corporation and Ladish Company have published their work on the effects of heat treating the nickel aluminide alloy IC-221M for 12 hours at 1204.degree. F. to 2200.degree. F. on the microstructural features in the eutectic phase and the gamma phase. Specifically, the changes of interest were the point at which the gamma phase started to coarsen and where the eutectic phase started to grow. They sought to demonstrate that Ni.sub.3 Al base alloys can be consumably remelted into production-size ingots without deleterious segregation or ingot cracking. This aforementioned work does not anticipate the instant invention where the mechanical properties of the nickel aluminide alloy IC-221M are increased through heat treatment to improve the alloy's performance in tooling and other structural applications.
In our search of the prior art, we found six articles of note. Their citations follow.
Liu, C. T., Stiegler, J. O. and Froes, F. H. "Ordered Intermetallics", Volume 2, ASM Metals Handbook, October, 1990, ASM International.
Han, Y. F., Li, S. H., Ma, S., Tan, Y. N. and Chaturvedi, M. C., "A DS Casting Ni.sub.3 Al Base Superalloy for Gas Turbine Blades and Vanes", The First Pacific Rim International Conference on Advanced Materials and Processing, 1992.
Izumi, O., "Intermetallic Compounds as Engineering Materials", The First Pacific Rim International Conference on Advanced Materials and Processing, 1992.
Orth, J. E., Sikka, V. K., "Commercial Casting of Nickel Aluminide Alloys", Advanced Materials & Processes, November, 1995.
Samuelsson, E., Keefe, P. W. and Furgason, R. W., "Evaluation of the Ni.sub.3 Al Base Alloys IC221 and IC218LZr", Superalloys, 1992.
Orth, J. E. and Sikka, V. K., "High Temperature Performance of Nickel Aluminide Castings for Furnace Fixtures and Components", 1996 Heat Treat Conference & Exposition, ASM International.